Spectral and Kinetic Study of 3-Styrylquinoxalin-2(1<i>H</i>)-ones Photoreduced by <i>N</i>-Phenylglycine and Amines

Abstract

The photoreduction by amines and N-phenyl-glycine, NPG, of six styrylquinoxalin-2(1H)-ones derivatives substituted in the styryl moiety, R-SQ was studied by using flash photolysis. The photoreaction is initiated via a single electron transfer from the electron donor (amines or NPG) to R-SQ excited triplet state, R-3-SQ*, with the formation of a triplet state radical ion pair or a charge transfer exciplex, (3)[CRIP/CTE]. These species live longer than the respective R-3-SQ* and have very similar transient spectra. In the presence of NPG, these (3)[CRIP/CTE] evolve on mu s time scale to the respective hydrogenated radicals, R-SQH(center dot), whose transient spectra and reaction rate constants with NPG are reported. The identity of these hydrogenated radicals was supported by the spectra obtained with the alpha-H donor triethylamine and previous pulse radiolysis studies in 2-propanol. Our findings allow proposing a radical chain reaction mechanism that explains the observed spectral behavior and rationalizes formation of the main product formed by binding of four PhNHCH2 center dot derived from NPG decarboxylation.